Ultrasonic transducer pairs, comprising a transmitter and a receiver, are commonly applied to detect air in a fluid delivery line segment as part of medication infusion systems, such as Plum A+™, Gemstar™ and Symbig™. The sensors are physically located on opposite sides of a fluid delivery line segment and the presence of air in the fluid delivery line causes an acoustical open circuit which substantially attenuates the detected signal. When fluid is present, propagation of the acoustic signal is efficient and produces a large electrical signal via the receiver circuit.
Detection of air in the fluid delivery line segment is typically performed on the basis of a fixed air-fluid boundary or threshold that is applied to the sensor voltage or current signal. Any signal on the fluid side of the threshold is classified as representing the presence of fluid in the infusion line and any signal on the air side of the threshold is classified as representing air. Typically a fixed threshold is applied that is specific to the infusion pump set and sensor arrangement.
When oscillating (bouncing) air bubbles are present in the fluid delivery line segment at the sensors of the infusion system, false air-in-line alarms may occur, due to the oscillating air bubbles continuously bouncing around the sensors thereby being repetitively counted and misleading the clinician into believing that the total air volume within the fluid delivery line has exceeded a total air threshold. This may cause the clinician to constantly stop the infusion system when it is not necessary.
When air is infused in the fluid delivery line segment past the sensors, the presence of a stationary fluid droplet (stuck fluid droplet) that bridges the gap between the sensors may lead to an acoustic short circuit. This can produce an absolute sensor signal similar to that of a fluid and result in a false negative indicating the presence of fluid when air is actually disposed over the sensor. This is problematic because the air sensor signal, indicating that air is in the infusion line, is typically used to produce an air-in-line alarm to pause the pumping mechanism and to reduce the potential for the infusion of air into a patient's vascular system. When a stuck fluid droplet is present at the sensors, the sensors may detect that fluid is present and as a result the air-in-line alarm may not be triggered even though air may be disposed in the fluid delivery line. This may create health issues for the patient.
Some infusion systems and methods have attempted to resolve the issues associated with bouncing air bubbles and stationary fluid droplets by developing complex algorithms to attempt to account for every potential situation in order to determine when air is in the system. However, it is virtually impossible to account for every situation through an algorithm alone.
An infusion system and method is needed which will improve the in-line detection of air in a fluid delivery line of an infusion system by reducing the problems associated with the presence of bouncing air bubbles and stuck fluid droplets in the fluid delivery line.